The major aim of this study is the characterization of the mechanism of Salmonella-mediated invasion of HeLa cells. A quantitative measure of this invasion has been established through the calculation of an infection index which clearly distinguishes between invasive and non-invasive bacterial strains. The factors which affect the invasive nature of a bacterium may be examined. Emphasis will be placed on the environment conditions that alter the parasite: host interaction, such as the atmospheric conditions, ion concentration, and presence of various chemicals in the infection system. Pathogenic strains of Salmonella are lysogenized with specific temperate phages which can be isolated. Such phages have been found to differ antigenically and in heat-sensitivity from Sal. typhimurium phage P22. Additional phage properties under consideration are 1) efficiency of transduction, 2) cross-immunity of lysogenized bacteria, 3) a definition of agents which block phage receptor sites and 4) the impact of lysogeny on the invasive nature of a pathogen bacterium. This last feature has particular significance since nonpathogenic strains of Salmonella lysogenized by these phages, but not P22, have an increased infection index. The nature of this lysogeny-mediated change will be considered by examining 1) culture filtrates and cell-free extracts for a component influencing the in vitro invasion process, 2) change in the antigenic composition of the bacterial host (via serological techniques), and 3) comparison of the specificity of the apparent lysogeny-mediated change by a variety of Salmonella phages on the infection index of non-pathogens.